He argues that two laws, not one, are involved. Detractors of the Atkins Diet often emphasize the first law of thermodynamics, that the total energy in a system is always conserved. Feinman considers the second law—that energy tends to dissipate over time—just as important. It determines what happens to the energy in food when it is broken down in the body. “We know on a molecular level that the body’s pathways are less efficient at turning protein calories into glucose,” he says. “That means that more energy is lost as heat.” So more of each calorie will convert into heat if it comes from proteins; more of the calorie will remain in the body if comes from carbohydrates.

Feinman emphasizes that his argument is solely rooted in physics. “I won’t pass judgment on the long-term health effects of any diet,” he says, “but with the epidemic of obesity we are facing, I think it’s premature to exclude low-carbohydrate diets from further consideration.”